An optical add/drop multiplexer that can add and drop light of different wavelengths is used in, for example, optical communication systems in which light having different wavelengths is transmitted by a single optical fiber. An optical add/drop multiplexer is also referred to as an OADM. In a submarine cable system, an optical add/drop multiplexer is installed in an optical branching unit.
FIG. 1 is a block diagram showing an optical submarine cable system that includes optical add/drop multiplexers.
In FIG. 1, optical branching unit 101 receives two optical signals on an uplink line: optical signal (hereinbelow referred to as “trunk signal”) 102a from trunk station 102 and optical signal (hereinbelow referred to as “branch signal”) 103a from branch station 103.
Optical signal 102a1 includes an optical signal of a wavelength that belongs to trunk signal band 102a and an optical signal of a wavelength that belongs to drop signal band 102a2. Trunk signal band 102a1 and drop signal band 102a2 are determined by wavelength. Trunk signal band 102a1 and drop signal band 102a2 have no overlapping portions (bands) each other.
Optical signal 103a includes an optical signal having a wavelength that belongs to add signal band 103a1. Add signal band 103a1 is further determined by wavelength. In addition, add signal band 103a1 and drop signal band 102a2 are in the same wavelength band.
Optical add/drop multiplexers 101a transmit, from among optical signal 102a, optical signals that belong to trunk signal band 102a1 and supply, from among optical signal 102a, an optical signal belonging to trunk signal band 102a1 toward trunk station 104. Optical add/drop multiplexers 101a further branch and do not transmit, from among optical signal 102a, an optical signal that belongs to drop signal band 102a2 and supply, from among optical signal 102a, an optical signal that belongs to drop signal band 102a2 toward branch station 103. In addition, optical add/drop multiplexers 101a add, from among optical signal 103a, an optical signal belonging to add signal band 103a1 to the optical signal belonging to trunk signal band 102a1 that was transmitted and supply the multiplexed optical signals toward trunk station 104.
The optical signal that is supplied to trunk station 104 from optical branching unit 101 (the multiplexed signal of the optical signal belonging to add signal band 103a1 and the optical signal belonging to trunk signal band 102a1) is received at trunk station 104 by way of optical repeaters 106. Optical repeaters 106 compensate loss of the optical signal due to the optical fiber in transmission line 105.
In the optical submarine cable system shown in FIG. 1, the above-described operation is also carried out on downlink lines.
In the optical submarine cable system such as shown in FIG. 1, in the event a no-input state in one of the two optical signals that are to be applied as input to one of the lines of optical branching unit 101 due to a fault such as a cable break, only the other optical signal (hereinbelow referred to as the survivor signal) is sent to the receiving station from optical branching unit 101.
Because optical repeaters 106 in a submarine cable system are operated by APC control (Automatic Pump Power Control), the output power of optical repeaters 106 is substantially fixed. As a result, when only a survivor signal is supplied from optical branching unit 101, this survivor signal is excessively amplified even more than in amplification carried out when there is an optical signal in a no-input state. As a result, the optical signal level per wave is increased, leading to deterioration of transmission characteristics due to a nonlinear optical effect.
In particular, when a cable breakage fault occurs in a cable between optical branching unit 101 and optical repeater 106 that is closest to optical branching unit 101, input (one optical signal) to optical branching unit 101 from the cable is completely cut off, and deterioration of the transmission characteristic of the survivor signal becomes prominent.
FIG. 2 shows how the signal level of another optical signal changes between a case in which one optical signal (signal B in FIG. 2) is present and the case of a no-input state. As shown in FIG. 2, when one optical signal enters a no-input state, the signal level of the other optical signal is higher than for a case in which one optical signal exists.
Patent Document 1 discloses an optical add/drop multiplexing system that can prevent excessive amplification of the other optical signal when one optical signal enters a no-input state. This optical add/drop system includes an optical amplifier unit, an output-power constant-control part, and an OADM unit.
Upon receiving the optical signal as input, the optical amplifier unit amplifies the optical signal. When an optical signal is not received as input, the optical amplifier unit supplies ASE (Amplified Spontaneous Emission) noise. ASE noise results from amplification by the optical amplifier unit of amplified spontaneous emission that occurs in the optical amplifier unit itself.
The output-power constant-control part monitors the output of the optical amplifier unit. The output-power constant-control part controls the optical amplifier unit such that the level of ASE that is supplied from the optical amplifier unit is the same level as the optical signal that is amplified by the optical amplifier unit when an optical signal is applied as input to the optical amplifier unit.
The OADM unit adds/drops an optical signal of a predetermined wavelength to the output light from the optical amplifier unit.
Essentially, this optical add/drop multiplexing system detects that an optical signal is not applied as input to the optical amplifier unit by monitoring the output of the optical amplifier unit. Then, upon detecting that an optical signal is not applied as input to the optical amplifier unit, this optical add/drop multiplexing system sets the level of ASE from the optical amplifier unit to the same level as the optical signal that is amplified by the optical amplifier unit when an optical signal is applied as input to the optical amplifier unit.
Thus, in a state in which an optical signal is not applied as input to the optical amplifier unit in this optical add/drop multiplexing system, ASE noise is used in place of the optical signal that is amplified and is supplied as output in the optical amplifier unit. As a result, the excessive amplification of the survivor signal can be prevented.
This optical add/drop multiplexing system is made up of an optical amplifier unit, an output-power constant-control part, and an OADM unit, these components being mutually independent.